Since a photographic process using silver halide is excellent in photographic properties such as sensitivity and gradation control as compared to other photographic processes such as electrophotography and diazo photographic processes, the silver halide photographic process has hitherto been most widely used. Recently, however, a technique capable of more easily and rapidly obtaining images has been developed by changing the image-forming process by a light-sensitive material using silver halide from a conventional wet process such as a process by a liquid developer to a dry process such as a developing process upon heating.
Heat developable photographic materials are known in the field of the art and these heat developable photographic materials and processes of processing them are described, in, for example, "Shashin Kogaku no Kiso (The Basis of Photographic Engineering)", pages 553-555, published by Corona K. K. in 1979; "Eizo Jooho (Image Information)", page 40, published on April 1978 "Neblett's Handbook of Photography and Reprography", 7th Ed., pages 32-33 (Van Nostrand Reinhold Company); U.S. Pat. Nos. 3,152,904; 3,301,678; 3,392,020; 3,457,075; British Pat. Nos. 1,131,108 and 1,167,777; and Research Disclosure, (RD-17029), pages 9-15, June, 1978.
Various processes have been proposed for obtaining color images. For example, for forming color images by a combination of the oxidation product of a developing agent and a coupler, there are proposed a combination of a p-phenylenediamine reducing agent and a phenolic or active methylene coupler in U.S. Pat. No. 3,531,286; p-aminophenol series reducing agents in U.S. Pat. No. 3,761,270; sulfoamidophenol series reducing agents in Belgian Patent No. 802,519 and Research Disclosure, pages 31 and 32, September, 1975; and a combination of a sulfoamidophenol series reducing agent and a 4-equivalent coupler in U.S. Pat. No. 4,021,240.
However, in these processes there is a fault that color images become turbid since images of reduced silver and color images are simultaneously formed at light exposed areas after heat development. For overcoming the fault, there are proposed a process of removing the silver images by liquid processing and a process of transferring the dyes only into other layer, for example, a sheet having an image-receiving layer. However, there remains a fault that it is not easy to discriminate a dye from the reaction mixture and transfer the dye only.
As an image-forming process of improving the foregoing fault, there is proposed a process of imagewise releasing a mobile dye by the oxidation reduction reaction with a light-sensitive silver halide and transferring the mobile dye onto a dye-fixing layer as disclosed in Japanese Patent Application (OPI) Nos. 58543/83; 79247/83; 149046/83; 149047/83, etc.
In the foregoing process of obtaining color images by a dry process, at the reduction of a light-sensitive silver halide into silver upon heating, a mobile dye is formed or released in chemical relation to the reaction but there is a fault that the reaction rate, that is, the heat developing rate during the process is slow. Also, in the foregoing process of obtaining color images by a dry system, an organic silver salt is sometimes used as one of the elements constituting a photographic material. However, in the case of dye-sensitizing such a photographic material by a sensitizing dye, the organic silver salt hinders the adsorption of the sensitizing dye onto the light-sensitive silver halide, which results in making difficult the performance of the dye sensitization.